Currently when you attempt to add a NetworkID for use with a user that has been previously input into the system you will get an object reference error stating an Item with the same key cannot be added.
Find within the database all instances of the NetworkID that are in use with a record or user and modify them to be different than the NetworkID you are attempting to input.
EXAMPLE: NetworkID testuser would be modified to testuser_deleted so that the new testuser can be added.
The following script can be modified by changing the '%testuser' at the end to to replace the testuser with the desired NetworkID
--This script is to show all instances of a NetworkID associated with a User.
--This will also list multiple domains using the same NetworkID as well.
--This will list the user and the associated NetworkID's
select tps_name, tps_network_login
join tps_user_network_login on tps_user.tps_guid = tps_user_network_login.tps_user_guid
where tps_network_login like'%testuser'
--Replace testuser with NetworkID that failed.
The NetworkID can be set by using the stored procedure from document - How to set Network Logins for your Users automatically which would run on a schedule and could be adding or modifying the Network login details for users nightly.